Chicago are celebrating the 50th anniversary of their first album this spring.

The two-LP set Chicago Transit Authority broke a lot of rules -- for length and content. As one of the first successful rock bands to build their music around a horn section, instead of just adding an accent or two, they stood out from the pack. The length of their songs was also unique.

The band's co-founders credit their producer James William Guercio for convincing Columbia Records that this had to be a double-album release.

They were geared to bring a distinct musical profile to rock 'n' roll. And 50 years later, there's no doubt they succeeded.

Sax player Walt Parazaider talked about the original blueprint for Chicago.

"About the time the group or the idea of the group was being formed, there was like James Brown was popular. Sam and Dave, Eddie Floyd 'Knock on Wood.' A lot of tunes that you're hearing re-recording today. We just started thinking about, 'What if we put a rock and roll band together... a rock and roll band with horns?' But not in a way that it would be, the horns would an afterthought -- they'd be an integral part of the group."

Horn player James Pankow discussed why Chicago Transit Authority had to be a two-LP set.

"Only reason we did a double album, and it's very simple, we needed two discs to encompass the total spectrum of the music. The average length of the cuts on the first couple of albums is like seven or eight minutes. Had we done one disc, there would have been four songs on the album. And James William Guercio again had to pull teeth."

"Terry Kath wrote that song after listening to Don Ellis - Live at Monterey. And he had a group that played at 32/4 time, 24/8 time and all of this. And it really was an introduction to the group, the first song on the Chicago Transit Authority album. It was sure a lot of fun to play. It was harder than heck. But it was influenced by Don Ellis, who has passed away since. But Terry was quite influenced by that. And I think it was a good introduction to the band. And we used to... for years, we opened with that song."

James Pankow discussed how horns were central to the sound of Chicago and not just mixed in as a sweetner.

"It's not just taking a little frosting off the cake; it's taking a big chunk of the cake away. It's not the fact that we had horns, it's to what extent we used them and how."

Walt Parazaider talked about Chicago's cover of "I'm a Man" and meeting the song's original artist.

"Spencer Davis heard that at the... we did it at the Royal Albert Hall and he was there to hear it -- and he thought that we did a better job of it than himself. I beg to differ. I thought we covered it quite well. But he had a nice young boy on there that sings, that still sings today. Stevie Winwood, that... quite an amazing talent."

Chicago Transit Authority helped the band earn a Grammy nomination for Best New Artist. And in 2014, the album was selected for the Grammy Hall of Fame.